ABSTRACT

It is known that microwaves with wavelengths ranging from 1 m to 1 mm or with frequencies between 300 MHz (100 cm) and 300 GHz (0.1 cm) are absorbed by the water vapor in the material. But in contrast to the radiation from infrared (IR) spectral range, where absorption is limited by the surface, microwaves can penetrate into deeper thicknesses of material. This means that the tested materials can be transparent to microwaves and thus the intensity of transmitted radiation will carry information about the amount of moisture in the sample investigated. Therefore, currently, this method is widely used in determining the moisture content in various materials. The analysis carried out in the present chapter shows that this approach can be successfully used not only to control the moisture in solids, but also for studies of atmospheric water vapor.

Already, weather satellites using IR sensors have widened our knowledge of the mechanisms of cloud formations, and through the use of IR spectrometers the vertical profiles of temperature and water vapor above the clouds have been probed. Thus, a great volume of important information was gathered. However, this information does not provide the necessary data concerning the lower troposphere, which are necessary for numerical weather prediction. The cloud-top limit effectively shields the majority of the lower atmosphere from infrared sensors. Even thin clouds are opaque to infrared radiation and restrict the region of the troposphere, which is to be explored by IR sensors. Analysis of hemispheric pictures of the Earth taken from the space for percentage of cloud cover has revealed how much of a handicap this is. Experiment and theoretical simulations have shown that the use of microwave region, which extends arbitrarily from millimeter to meter wavelengths, gives possibility to circumvent this limitation. A description of microwave hygrometers that can be used for this purpose is given in this chapter.